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Amino Acid Geochronology Laboratory

Charophyte oogonia and amino acid geochronology

A scanning electron micrograph of an oogonium. The carbonate shell is broken, revealing the oospore.

The summer monsoon is one of the primary sources of moisture over the semi-arid regions of central Australia. Often, dry lakes in the interior of Australia are used to reconstruct the timing and the intensity of monsoons in the past. Investigation of the stratigraphy surrounding these lakes shows that elevated shoreline deposits demarcate these prior highstands. A lack of suitable material for dating techniques has hampered our understanding of the exact timing of these monsoonal shifts.

Here I've used charophyte oogonia, the calcified algal fruiting bodies found in shallow lacustrine sediments, as a new sample material for amino acid geochronology. Improved instrumentation now allows the use of extremely small samples (~0.01 mg), reducing the required sample size and therefore enabling higher sampling precision of lacustrine sediments (e.g. beaches, lake cores). An assessment of sample quality regarding charophyte oogonia was made and several applications of amino acid racemization using these algae were explored. The majority of the oogonia analyzed in this experiment displayed random D/L variability between subsamples from the same stratum despite high levels of analytical reproducibility (< 8% variation). The concentration of amino acids in oogonia was investigated as a potential source of this D/L variability, but there was no correlation between the two parameters. It is this variability which hampered nearly all chronologic reconstructions based on amino acid racemization.

The purple dots on the map show general sampling regions for the charophyte study. The two southernmost dots were locations containing modern samples (Coorong Nat'l Park and lakes near Robe). The three northward sites primarily contained fossil oogonia. Hundreds of samples were collected from these locations, most of which have been analyzed via reverse-phase liquid chromatography.

A satellite image of Lake Eyre in flood. During several periods of increased monsoon intensity over the last 125,000 years, Lake Eyre contained at least this much water perennially.

A satellite image of a dry Lake Eyre. Once the lake dries after an extended period of lake-full status, sediments are left behind which often contain charophyte oogonia.